Nature is organized in such a way that a large portion of the solar energy absorbed by the surface of the earth in tropical and subtropical areas is converted into wind energy. As such, it is then transported to the northern hemisphere, for example, to Europe, the Atlantic, the North Sea, etc. Per square meter of surface, the wind constitutes a force which corresponds, on average, to a power of approximately 1 kilowatt. One can utilize this energy by means of modern wind power stations. However, because of the fact that the wind doesn't blow all the time, an industrial society can use wind power as a reliable energy source only if the electric energy thus generated can be stored to a sufficient extent and for a period of at least hours, possibly days. The same applies to electric energy generated by photovoltaic systems.
If such storage can be achieved on a large scale, these regenerative sources of energy could be better used to cover the basic load, and one day in the future they might entirely replace fossil fuels and nuclear power.
Traditional storage techniques for electricity, such as conventional or rechargeable batteries, can store only a tiny fraction of the required energy. Additionally, they are very expensive and therefore on a grand scale not acceptable from an economic point of view. Because of heat loss, the chemical storage methods frequently discussed today (for example, electrolysis of water) and compressed air storage show a relatively poor efficiency in recovering the stored energy. An efficiency in the order of about 30 percent would already be good values for such types of storage.
By today's standards, only water pumped storage power plants (pumped-storage hydroelectricity, PSH) remain useful for efficiently storing large amounts of electric energy. These can recover the stored energy with an efficiency of approximately 80 percent. In these plants, at times of excess electricity, water—mostly from artificial lakes—is pumped from a lower reservoir to an upper reservoir. The larger the reservoirs and the greater the height difference, the more energy can be stored. At times of increased power demand, the water which has been pumped up as described is allowed to flow through turbines back into the lower reservoir. In this process the difference of the potential energy of the water is converted into electric energy. The power W depends on the product of the height difference h between both reservoirs times the water flow rate M. Assuming a water density of 1000 kg/m3, the following simple formula applies: W (kW)=9.81×M (m3/s)×h (m). This results in a total capacity of the energy storage facility of E (kWh)=9.81×M×h×t/3600 (hours). t is the maximum period of time for lowering the water level in the upper reservoir in hours. In a pumped storage power plant, the pumping, storage and power generation phases are constantly alternating. Pumped storage power plants can be started quickly and can therefore promptly react to increased power demand. Currently, Germany has approximately 30 pumped storage power plants. Because of the required height differences, they are located in medium or even high mountain ranges. The largest plants in Germany can be found near Goldisthal, Thuringia (power approximately 1 gigawatt and energy storage capacity approximately 8.5 GWh with a useful volume of approximately 12 million cubic meters) and Markersbach, Saxony (power approximately 1 gigawatt and energy storage capacity approximately 4 GWh). Altogether, the power of all pumped storage power plants in Germany amounts to almost 7 gigawatts.
However, the demand for such pumped storage power plants exceeds the potential typically available. Energy storage facilities close to power production plants are highly desirable, for example, in the case of offshore wind energy. Therefore, it is an important economic task to develop these capacities (see, for example, Energie-Forschungszentrum Niedersachsen [Energy Research Center, Lower Saxony], Goslar, http://www.efzn.de). Currently, the construction of new pumped storage power plants in the mountains and in abandoned mines is being discussed as a possibility for developing these capacities. The use of underground mining sites requires large-scale above-ground water storage. Such construction often fails due to existing residential developments or other preexisting uses. Moreover, the existing empty volumes in mining sites are small and distributed over long underground conduits, making it difficult to realize efficient power stations. Furthermore, storage sometimes requires long power transmission routes and severe intervention in the natural environment. Generally speaking, the global availability of suitable locations for such pumped storage power plants is limited.
Therefore, we present a completely different approach for new pumped storage power plants which, at first glance, may perhaps seem unrealistic.